There are 86400 seconds in a day.

The statement "there are 86400 seconds in a day" is true because there are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day. Therefore, if we multiply these units together (60 seconds/minute * 60 minutes/hour * 24 hours/day), we will get the total number of seconds in a day, which is 86400 seconds.